There are different modulation methods for transmitting data. One possible modulation scheme, particularly in optical data transmission systems, is known as differential phase shift keying, or DPSK for short, whereby the information is encoded in the phase difference between two consecutive bits of a data signal. One possible implementation is to leave the phase of the subsequent bit the same if a logical “one” is to be transmitted and to change it by 180° if a logical “zero” is to be transmitted or vice versa.
To receive and convert the phase information into receiver-end amplitude information, i.e. to recover the data signal, in the receiver an interferometer, for example, is used at whose outputs the light of a transmitted bit is heterodyned with that of the subsequent bit. Normally employed for this purpose is a Mach-Zehnder arrangement wherein the path difference in the two branches or arms of said Mach-Zehnder arrangement corresponds to precisely one bit period. One output of this Mach-Zehnder arrangement or rather Mach-Zehnder interferometer emits light if the phase of the two interfering bits is identical. Another second output of the Mach-Zehnder arrangement has light if the phase is 180° shifted.
It is advantageous to use the information at both outputs of the Mach-Zehnder arrangement by connecting the two outputs to photodiodes and processing the signals of said photodiodes in a differential amplifier, thereby achieving in practice a sensitivity gain of just under 3 dB, i.e. for the same bit error rate a 3 dB lower optical signal-to-noise ratio OSNR is required. This is an advantage of DPSK modulation compared to other methods.